The present invention concerns a miniaturized sensor component for measuring concentrations of substances in liquids. The sensor component comprises a support. The support contains cavities. The cavities accommodate a membrane. The membrane is produced in situ by polymerization or by precipitation from a solution. An electric lead is either mechanically connected to or integrated into the membrane and is in contact with the membrane.
The present invention also concerns a method of manufacturing such a sensor component.
The membranes in miniaturized sensor components of the aforesaid genus are ion-selective. The sensors are employed in medicine to measure blood-electrolyte levels and in environmental engineering to determine how contaminated drinking water might be.
Manufacturing miniaturized ion-selective electrodes (ISE's) by the "wire-coating" method is known (P. Berveld, Development and Application of Chemical Sensors in Liquids, in Sensors and Sensory Systems for Advanced Robots, Berlin and Heidelberg, Springer, 1988, 403). A slender silver wire is coated with an ion-selective polymer membrane obtained from a liquid by immersing the wire in a solution for example.
Ion-selective electrodes can also be manufactured by coating a sheet of plastic instead of wire (U. Lemke & K. Cammann, Coated Film Electrodes, Fresenius Z. Anal. Chem. 335 [1989], 852-54). In their simplest form, these electrodes comprise an ion-selective polymer membrane over a thin layer of silver on the plastic.
Methods for producing such ion-selective polymer membranes are also known (Fluka Feinchemikalien Gmbh, Neu-Ulm sales literature, Selectophore Ionophores for Ion-Selective Electrodes). Such a membrane typically consists of a polyvinyl-chloride matrix containing both a softener and an electrically active substance (an ionophore) that determines what ions the membrane will allow through. The substance, dissolved in such a solvent as tetrahydrofuran, is cast into a membrane that solidifies as the solvent evaporates.
Slightly more complicated than the aforesaid sensor components are what are called thin-film electrodes (K. Cannann et al, Chemo- und Biosensoren-Grundlagen und Anwendungen, Z. Angew. Chem. 103 [1991], 519-541]. These components have a layer of silver chloride between the elemental silver and the ion-selective polymer membrane as well as a solidified layer of reference electrolytes. The reference electrolytes make it possible to establish a constant potential difference between the membrane and the silver.
The potential at the phase boundary between the solution being tested and the ion-selective membrane in such an electrode will vary with the ionic activity of the solution and hence with its concentration of ions. The variation can be compared with a reference electrode of constant potential.
Several ion-selective membranes can be combined on a common support to make a multiple sensor (European Patent Application No. 0,302,228 A2).
Using ion-selective electrodes to make biological sensor components is also known. An enzyme is established in the ion-selective membrane, at its surface, or in or on another membrane or layer of gel by adsorption, gel inclusion, covalent coupling, or cross-linking. Glucose oxidase for instance can be employed to manufacture potentiometric glucose sensors (F. Scheller & F. Schubert, Biosensoren, Berlin, Akademie-Verlag, 1989, 89 & 49-52).
The known sensor components have drawbacks. Thin-film electrodes for instance are complicated to manufacture. Coated-sheet electrodes are easier to make by casting or cementing, but each sensor component is made, and must accordingly be calibrated, separately.